In short, the hidden sex tag is mostly used to describe sexual intercourse in places where the parties involved need to be sneaky, silent, quiet or quick so as they aren't discovered. Thus I think the description formerly proposed aren't up to the real meaning of the tag, with all due respect.

I think "Sexual activities that take place in places or situations where there is a real risk of being noticed." is better than 1. "Sex scenes that take place secretly or unnoticed by others in a typically public place." - because a) When a husband and wife have sex in the same room as their child, b) when a Wife is fucked over her sleeping / passed out husband, c) when 2 people are touching each other below the kotatsu or d) when a couple have sex in the front yard during the day. are not in a public space but are still "hidden" and are always tagged as hidden sex. 2. "Sex scenes that take place secretly or unnoticed by others", because a) sex in the comfort of your bedroom during the night is taking place unnoticed by others but it's not hidden; b) sex between coworkers in a love hotel or their houses can be a secret from the coworkers if the relationship is not publicly known by their choice but it's hardly hidden as well.

support @K14M's description most

What should remain unseen is shown and yet still unseen by some.

Sex scenes that take place secretly or unnoticed by others in a typically public place. Similar to exhibition, but without being seen. Or, just secretly, like trying not to get caught or seen by roommates/ parents.

I always saw this "sex in plain sight" as opposed to exhibitionism which is typically outdoors this is like under the blanket covers or sitting casually on someone's lap on a train, portraying normal situations while sex is actually happening. Getting thrilled from being seen without others knowing.

Sex scenes that take place secretly or unnoticed by others